Manganese is a metal which is not found in nature as a free form, but rather combined in minerals in the composition of which it is found as an oxide (primarily pyrolusite) or carbonate (primarily rhodochrosite). Production processes traditionally requiring the addition of manganese have been carried out using these minerals in iron metallurgy reduction furnaces for the purpose of using the manganese as an alloy element or as a deoxidizing and desulphurizing agent in steel production. However, at the end of the 19th century, different methodologies for obtaining pure manganese from manganese minerals began to be studied for the purpose of improving the alloys in which it participated and for broadening its field of application, several processes being developed: aqueous electrolysis of manganese salts, electrothermic, carbothermic, aluminothermic and silocothermic processes.
Among the background, U.S. Pat. No. 2,511,507 for “TREATING MANGANESE ELECTROPLANTING SOLUTIONS”; U.S. Pat. No. 2,483,287 for “METHOD OF PURIFYING MANGANESE ELECTROLYTES”; U.S. Pat. No. 2,347,451 “ELECTROLYTIC DEPOSITION OF MANGANESE”; U.S. Pat. No. 2,343,293 “PROCESS FOR THE PURIFICATION OF MANGANOUS SULPHATE SOLUTIONS”; and U.K. patent 528.112 “IMPROVEMENTS IN THE ELECTROLYTIC PRODUCTION OF MANGANESE” are worth mentioning.
Of all of these processes, the one which allows obtaining greater manganese purity, has very accessible production costs and, therefore, is the most used process, is the first one, i.e. the production of manganese by means of aqueous electrolysis of salts of this metal or obtaining electrolytic manganese. This product is currently marketed with purities ranging between 99.5 and 99.9% metal.
The electrolytic route for obtaining manganese metal was first researched by Davis in 1930. However, this process did not become important until 1939, when the demand for electrolytic manganese by steel producers (for the manufacture of weapons) forced the U.S. Bureau of Mines to install a pilot plant in Knoxville (Tenn.). This plant was redesigned in 1940, and in 1944 it reached a production capacity of 1,500 t/year. In Japan, electrolytic manganese production began in 1941. The U.S. Bureau of Mines built a second pilot plant in 1942 in Boulder City. For its part, The Electrolytic Manganese Corporation of Krugersdorp, in South Africa, began production in 1955. Currently, most of the electrolytic manganese consumed worldwide is produced in China and South Africa.
The process for obtaining electrolytic manganese which has been developed at a laboratory level stems from the need to solve the greatest environmental drawback derived from the manufacture of ferroalloys for any other industrial waste having magnesium in general, which is the generation of waste as a result of the processing of the production furnace exhaust fumes. The most effective manner of processing these emissions is wet gas scrubbing, such that the particles contained therein are retained in water. The subsequent processing of this water gives rise to obtaining a waste product having a high manganese content which is difficult to use as a recyclable material due to its physical nature.
The drawback described made the requesting company assign the Cátedra de Metalurgia de la Universidad de Oviedo (Spain) four years ago the task of carrying out viability tests for the extraction of the manganese content of this waste by hydrometallurgical route for its subsequent electrolytic recovery, and to thus obtain a product of high added value. The research carried out has clearly shown that by means of certain novelties on the previously mentioned methods, it is possible to meet this challenge.